For recovering gold and/or other precious metals from ores, a number of lixiviant systems have been proposed and used over the past century. The word "lixiviate" means to extract a constituent from a solid mixture. A lixiviant system is one that contains the components necessary to extract the desired constituent. The most widely used lixiviant system for gold is a combination of sodium cyanide as ligand together with air (oxygen) as oxidant. Hydrogen peroxide is sometimes used as an auxiliary oxidizing agent. Ores that are resistant to simple extraction or lixiviation procedures are commonly referred to as "refractory" ores.
Many gold deposits in rock were created by the precipitation of gold along with sulfide minerals during the flow of hydrothermal fluids through the rock. Depending on the deposition mechanism, the sulfide minerals can be present alongside the gold or can physically encapsulate it. Over time, the zone of such deposits nearest the earth's surface will have been oxidized by weathering, and the sulfides so oxidized carried away by groundwater flow. This zone is referred to as the "oxide zone". In the deepest portions of the deposits, below the water table, the sulfide minerals remain more or less in the form in which they were deposited. This zone is referred to as the "sulfide zone". The relative size of these zones is determined by the depth of the deposit, historical water table fluctuations and surface weathering conditions, among other factors.
Where the sulfide minerals persist in such a gold deposit, they demonstrate varying degrees of reactivity to sodium cyanide, the chemical lixiviant commonly used in gold leaching, and to oxygen, consuming them and requiring the addition of fresh materials. While some iron-containing sulfide minerals such as pyrite and chalcopyrite exhibit relatively low reactivity during the time span of most gold lixiviation processes, others such as pyrrhotite are highly reactive. The added processing cost due to consumption of lixiviant chemicals by a high concentration of these highly reactive minerals can make recovery of portions or all of a gold deposit uneconomic.
In the case of gold deposits where the gold is physically encapsulated in the sulfide minerals, the minerals can create a surface barrier, which prevents the gold from being extracted. In this case, procedures such as roasting, pressure oxidation or biological oxidation of the deposit can be employed. Such procedures are very capital-intensive and costly.
In cases where the sulfide minerals do not physically block the access of the lixiviant solution to the gold, that is, the minerals are present with the gold but do not encapsulate it, an excess of lixiviant can be used, or the gold deposit can be pretreated in some way to passivate the surface of the sulfide minerals to make them less reactive to the lixiviant solution.
Certain nickel and cobalt ores also contain iron-containing sulfidic minerals such as pyrrhotite, making the ores unsuitable for cyanide leaching.
A closely related problem, known as acid rock drainage, occurs in the case of iron-containing sulfidic materials resulting from mining and leaching of various metallic and non-metallic minerals, including gold ores. These sulfidic materials include, but are not limited to, tailings, overburden, discarded waste rock removed along with ore, and unmined exposed rock such as in pit walls. The natural air/water oxidation processes described previously in relation to the surface layers of a gold deposit (the oxide zone) will also occur with these materials, causing the formation of sulfuric or related acids. These acids are the cause of severe pollution problems throughout the world. Similar problems occur with the exposed surfaces of coal mines.
U.S. Pat. No. 5,587,001 discloses a process for pretreating sulfidic iron-containing ores prior to lixiviation by contacting the ores with an aqueous solution containing manganate ions or precursor of manganate ions at a concentration between 0.0005 mole % and saturation, allowing a precursor to react to form manganese ions, and maintaining the solution pH between 6 and 13 so as to form a layer of manganese oxide on the surface of the sulfides. In scaling up this process, however, it has been found to be only marginally effective in certain cases, presumably because acid generation by the ore prevents sufficient passification of the ores' reactive surfaces.
As a result, there is a need to develop a process for effectively pretreating refractory gold or other metal deposits, which contain iron-containing sulfidic minerals that do not encapsulate the gold, to reduce the consumption of lixiviant chemicals. There is also a need for pretreating other metal deposits which contain such sulfidic minerals. While the present application centers on ores containing precious metals such as gold and silver, platinum, nickel, cobalt and other metals are also amenable to such the present invention process.